I. Introduction
This invention relates to a process for electroplating the surface of a nonconductor by forming an adherent, chemically resistant, precious metal sulfide conversion coating over the surface of the nonconductor which coating functions as a base for direct electroplating.
II. Description of the Prior Art
Nonconductive surfaces are often metallized by a sequence of steps comprising catalysis of the surface of the nonconductor followed by contact of the catalyzed surface with an electroless plating solution that deposits metal over the catalyzed surface in the absence of an external source of electricity. Following electroless metal deposition, the metal deposit is optionally enhanced by electrodeposition of a metal to desired full thickness.
Early catalyst compositions and processes for electroless metal deposition are disclosed in U.S. Pat. No. 2,702,253 incorporated herein by reference. The process comprises a sequence of two steps where the first step is immersion of a part to be plated into an acidic solution of stannous chloride. The part is then rinsed and dipped into an acidic solution of platinum group metal. The stannous chloride retained on the surface of the part provides the dual function of reducing the platinum group metal to a metal and bonds the reduced platinum group metal to the surface to be plated. The reduced platinum group metal is catalytic to deposition when in contact with an electroless metal plating solution resulting in electroless deposition over the catalyzed surface.
Problems are encountered using the two step catalyst of U.S. Pat. No. 2,702,253 for the formation of printed circuit boards because the platinum group metal, in contact with the copper cladding of a circuit board substrate, forms a platinum group metal immersion deposit that interferes with adhesion of a subsequently applied electroless metal to the copper.
An improvement in catalysis technology is disclosed and claimed in U.S. Pat. No. 3,011,920 incorporated herein by reference. The catalysts of this patent consist of an aqueous suspension of a tin - noble or precious (catalytic) metal colloid. By immersion of a part to be plated in the catalyst, colloid is adsorbed onto the surface of the part. Subsequent contact of the part with an electroless plating solution containing dissolved metal and reducing agent in solution results in plate out of metal in contact with the adsorbed catalytic metal - tin catalyst.
As is known in the art of electroless plating, the presence of a dissolved metal and a reducing agent together in solution can result in solution instability and indiscriminate deposition of metal on the walls of containers for such plating solutions. This can result in the requirement to shut down a plating line and replace the plating solution.
Attempts have been made to eliminate the need for an electroless plating solution using a direct plating sequence whereby a metal is electrolytically deposited directly over a treated nonconductive surface. One such process is disclosed in U.S. Pat. No. 3,099,608, incorporated herein by reference. The process disclosed in this patent involves treatment of the nonconductive surface with a tin palladium colloid. This is the same tin palladium colloid used as a plating catalyst for electroless metal deposition. It is possible to electroplate directly over the catalyzed surface of the nonconductor from an electroplating solution though deposition occurs by propagation from a conductive surface with deposition beginning at the interface of a conductive surface and the catalyzed nonconductive surface. The deposit grows epitaxially along the catalyzed surface from this interface. For this reason, metal deposition onto the substrate using this process is slow and the deposit often suffers thickness variation.
An improvement in the process of U.S. Pat. No. 3,099,608 is disclosed in U.K. Patent No. 2,123,036B. In accordance with the process described in this patent, a surface is provided with metallic sites and the surface is then electroplated from an electroplating solution containing an additive that is said to inhibit deposition of metal on the metal surface formed by plating without inhibiting deposition on the metallic sites over the nonconductive surface. In this way, there is said to be preferential deposition over the metallic sites with a concomitant increase in the overall plating rate. In accordance with the patent, the metallic sites are preferably formed in the same manner as in the aforesaid U.S. Pat. No. 3,099,608--i.e., by immersion of the nonconductive surface in a solution of a tin palladium colloid. The additive in the electroplating solution responsible for inhibiting deposition is described as one selected from the group of dyes, surfactants, chelating agents, brighteners and leveling agents. Many of such materials are conventional additives for electroplating solutions.
There are limitations to the above process. Both the processes of the U.S. and the U.K. patents for direct electroplating require conductive surfaces for initiation and propagation of the electroplated metal deposit. For this reason, the processes are limited in their application to metal plating of nonconductive substrates in areas in close proximity to a conductive surface.
One commercial application of the process of the U.K. patent is the metallization of the walls of through holes in the manufacture of double-sided printed circuit boards by a process known as panel plating. In this application, the starting material is a printed circuit board substrate clad on both of its surfaces with copper. Holes are drilled through the printed circuit substrate at desired locations. For conductivity, the hole walls are catalyzed with a tin palladium colloid to form the required metallic sites on the surfaces of the walls of the through holes. Since the circuit board material is clad on both of its surfaces with copper and the circuit board base material is of limited thickness, the copper cladding on the surfaces of the circuit board material is separated by the thin cross section of the substrate material. The next step in the process is direct electroplating over the catalyzed hole walls. Since the copper cladding on each surface is separated by the cross section of the substrate, during electroplating, deposition initiates at the interfaces of the copper cladding and the through hole walls and rapidly propagates into the holes. The hole wall is plated to desired thickness within a reasonable period of time. Thereafter, the circuit board is finished by imaging and etching operations.
A disadvantage to the above panel plating process is that copper is electroplated over the hole wall and over the entire surface of the copper cladding. The steps following plating involve imaging with an organic coating to form a circuit pattern and removal of copper by etching. Therefore, copper is first electrolytically deposited and then removed by etching, a sequence of steps which is wasteful of plating metal, etchant and time, and therefore, more expensive.
The art has developed a method for manufacture of printed circuit boards known as pattern plating. In this process, a printed circuit board base material is drilled at desired locations to form through holes. The through holes are metallized using conventional electroless plating techniques. Electroless copper is plated onto the walls of the through holes and over the copper cladding. Thereafter, photoresist is applied and imaged to form the circuit pattern. The board is then electroplated with copper depositing on the copper conductors and through hole walls, but not over the entire surface of the copper cladding. Solder mask is then plated over the exposed copper by immersion or electroplating and the remaining photoresist is stripped. The copper not protected by the solder is then removed by etching to form the copper circuit.
Pattern plating cannot be used with the metallizing process of the aforesaid U.K. patent. The treatment of the copper cladding prior to the application of the photoresist and the development of the photoresist, all as required for pattern plating, requires the use of treatment chemicals found to dissolve or desorb the tin palladium colloid from hole walls. Since this occurs prior to electroplating, direct electroplating to provide conductive through holes becomes impossible.
A further improvement in the art is disclosed and claimed in copending U.S. patent application Ser. No. 07/071,865 filed July 10, 1987 (now abandoned) and assigned to the same assignee as the subject application. According to the process of the this application, an electroless plating catalyst, such as that disclosed in the aforesaid U.K. patent, is treated with an aqueous solution of a sulfide, such as a sulfur solution, to convert the catalytic surface to a sulfide surface. By conversion of the surface to the sulfide conversion coating, the electroless plating catalyst is not desorbed from the surface during metallization, and consequently, in accordance with the process of said application, it is possible to pattern plate substrates in the formation of printed circuit boards.